Irrigation systems such as center pivot and linear irrigation systems have a number of irrigation spans each supported by a mobile support tower powered by a drive motor. The drive motors are typically single speed motors that are frequently switched on and off for different lengths of time to move the irrigation spans at different rates of speed. The single speed drive motors accelerate and decelerate very rapidly when switched on and off, which applies significant loads and torques to the irrigation spans and the drive trains connecting the drive motors to the wheels of the mobile support towers. The high torques also cause the irrigation spans to oscillate back and forth, thus imposing cyclic loads on the irrigation spans and drive trains. The oscillations also cause false alignment faults between adjacent irrigation spans, thereby re-activating the drive motors and creating additional oscillations. The oscillations can propagate to other irrigation spans, thus forming a standing wave across several irrigation spans. This adds unnecessary structural fatigue cycles to the irrigation spans and drive trains and more quickly wears out electrical contacts, sensors, and other electrical components. These problems can be minimized by using variable speed motors to drive the towers, but variable speed motors are too expensive for many applications and have other limitations.